Carburetor anti-dieseling and deceleration control

ABSTRACT

The carburetor air/fuel idle system mixture channel has an air port that is normally closed by a manifold vacuum sensitive valve; during either engine deceleration or shutdown operations, the air valve is opened to bleed the vacuum signal on the idle system fuel jet, to terminate idle system fuel, air flowing through the idle system air/fuel mixture channel to the engine cylinders.

I Unlted States Patent [1 1 I111 3,795,237

Denton 1 Mar. 5, 1974 [54] CARBURETOR ANTI-DIESELING AND 2,439,573 4/1948 Mallory 123/97 B DECELERATION CONTROL 3,577,966 4/1971 Callingwood et a1, l23/DIG. 11

2,390,603 12/1945 Mallory 123/97 B [75] Inventor: Ronald T. Denton, Birmingham,

Mich. [73] Assignee: Ford Motor Company, Dearborn, Primary Exammer wende Bums Mich.

[22] Filed: Dec, 3, 1971 {57] ABSTRACT [21] Appl. N0.: 204,527

The carburetor air/fuel idle system mixture channel has an air port that is normally closed by a manifold [52] 123/198 123/DIG' 123/97 B vacuum sensitive valve; during either engine decelera- I51] f 7/06 Fozd 31/00 F02m 7/00 tion or shutdown operations, the air valve is opened to [58] new of I23/DIG- 1 97 198 BB, bleed the vacuum signal on the idle system fuel jet, to 23/198 DC terminate idle system fuel, air flowing through the idle I I systemair/fuel mixture channel to the engine cylin- [56] References Cited dew UNITED STATES PATENTS 3,635,203 1/1972 Gannoe .l.., 123/DIG. 11 9 Claims, 1 Drawing Figure 2 Q50 24 Z6 T I a I I I I I I I I I 1 i I I I l 2a 22 I I ,L J I I Jae. I A. 40 -/6- I W I A l I I 9 t M in") CARBURETOR ANTI-DIESELI'NG AND DECELERATION CONTROL This invention relates in general to a carburetor for an internal combustion engine. More particularly, it relates to the control of fuel flow through the idle system of a carburetor to prevent engine dieseling after engine ignition shutoff, and also to prevent the emission of undesirable elements into the atmosphere during deceleration of the engine.

Carburetor constructions are known in which valving is moved into the carburetor idle system channel to completely block the flow of air and fuel through it to the engine during certain phases of operation, such as, for example, during deceleration. During this latter phase, the extremely high manifold vacuum force interferes with proper scavenging of the engine cylinders, resulting in incomplete combustion and the possible emission of undesirable elements into the atmosphere.

I it is a primary object of this invention to provide a carburetor construction such thatduring engine decelerating conditions and upon engine shutdown, the idle air/fuel mixture channel'fuel supply is terminated to stop combustion, and" is replaced by airflow to better scavenge the engine cylinders atv this time and lean whatever mixture may be present in the engine cylinders.

it is a further object of the invention to provide a carburetor constructed with an-idle air/fuel mixture channel that has an air port connected to'the air cleaner and opened and closed by a manifold vacuum sensitive valve that is responsive to engine shutdown and deceleration operations to automatically terminatethe idle channel fuel supply and replace it with a charge of fresh air at essentially atmospheric pressure flowing to the engine cylinders.

it is a still further object of the invention to provide a carburetor constructed with a first manifold vacuum controlled valve to automatically bleed air into the idle air/fuel mixture channel when the engine shuts down so as to terminate idle channel fuel flow;.and, a second manifold vacuum controlled valve that is operable during engine deceleration to bleed airto the vacuum line controlling the movement of the first valve whereby the idle channel fuel vacuum signal will decay to terminate idle system fuel flow and at the same time air. will flow throughthe idle channel towards the engine cylinders.

Another object of the invention is to provide a carburetor constructedwith an idle air/fuel mixture channel having an air bleed port opened or closed in response to the movement of a manifold vacuum controlled valve that opens the port upon engine shutdown to decay the idle system fuel vacuum signal while at the same time admitting air to the engine cylinders through the idle channel, the valve being subsequently closed automatically after a predetermined time delay to recondition the idle channel to be subject to the conventional manifold vacuum signal.

Other objects, features and advantages of the invention will become more apparentupon reference to the succeeding detailed description thereof, and to the drawing illustrating a preferred embodiment thereof; wherein the single FIGURE illustrates schematically a cross sectional view of a carburetor constructed according to the invention.

TheFlGURE shows a portion 10 of the intake manifold of an internal combustion engine. Opening into the manifold is the lower end 12 of an induction passage 14 of a downdraft type carburetor 16. The carburetor has an air inlet 18 at its upper'end mating with the discharge end of a dry element type air cleaner assembly 20.

The air cleaner has the usual dish-shaped lower tray portion 22 with a central discharge aperture coinciding with the inlet 18 to carburetor 16. The open top portion of the tray 22 is closed by an annular cover 24. A snorkel like air inlet tube or duct 26 is aligned with a hole in one side of the tray 22 for the passage of fresh air radially through an annular pleated paper or other similar type filter element 28.

Induction passage 14 has the usual fixed area venturi 30 within which may be located one or more booster venturis, not shown. The main flow of fuel through the carburetor usually enters through the booster venturi via a line not shown connected to the conventional fuel float bowl, also not showmFlow of air and fuel through induction passage 14 is controlled by the movement of a throttle valve plate 36 fixed on a shaft 38 rotatably mounted in the side walls of the carburetor bore.

The carburetor has the usual idle system fuel/air mixture channel 40 supplementing the main fuel metering system and bypassing the throttle valve. It has a conventional transfer slot or port 42, and a discharge port 44 located below the closed position of throttle valve 36. The discharge area of port 44 is varied by an adjustable needle valve 46, in a known manner.

' Further details of construction and operation of the main and idle fuel and air systems is not given since they are known and believed to be unnecessary for an understanding of the invention. Suffice it to say, however, that the idle system would include the usual fuel jet and air and air/fuel orifices, the air orifice being connected to the air horn portion of the induction passage. The vapor space above the fuel in the float bowl would be connected by a passage to' the air horn section of the'carburetor or to the air cleaner proper so as to equalize the top of the fuel in the tank with the air horn essentially atmospheric pressure.

Thus, when the engine is running, the force of the manifold vacuum acting on the dischargeport and area of the transfer slot beneath the closed throttle valve edge creates a pressure differential in the idle channel across the orifices and the fuel jet. This causes fuel and air to be drawn through the idle channel and out the discharge port to the engine cylinders, in a known manner.

It is this very flow of fuel through the idle channel that is objectionable during both engine decelerating operating conditions and during engine shutdown. During the former time, incomplete'combustion and improper scavenging of the engine cylinders occurs so that the emission-of undesirable elements such as uncylinders to lean whatever mixture may be present therein and also efficiently scavenge the cylinders.

More specifically, the idle down channel has a pair of air ports 48 and 50 that are supplemental to those generally provided. The ports are connected by suitable lines 52 and S4 to an air chamber 56. Chamber 56 in turn is connected by a line 58 to the clean side of the air filter element 28.

lt will be clear that the flow of this additional air through lines 52 and 54 into idle channel 40 will destroy the pressure differential across the idle channel fuel jet, not shown, so that idle channel fuel flow will then terminate. However, it'shouldbe noted that the engine suction acting on the discharge port 44 and that area of the transfer slot 42 below the edge of throttle valve 36 will cause air flow through the idle channel into the engine proper, thereby leaning out whatever mixture may be present in the engine and also scavenging the engine cylinders of residual gases, etc.

A valve 62 is slidably mounted to open or close the air lines 52 and 54, and is adapted to be moved by a manifold vacuum sensitive servo64. The latter consists of a hollow shell 66 divided into two vacuum chambers 68 and 70 by an annular flexible diaphragm 72 secured to the stem 74 of valve 62. A spring 76 normally biases the diaphragm and valve to a closed position blocking the communication of air from the air cleaner to lines 52 and 54.

Chamber 70 is connected by a line 78 to a manifold vacuum port 80 in induction passage 14 through a vacuum reservoir 82 and an orifice or flow restriction 84. The orifice 84 provides a slow build up and decay of any vacuum that is in reservoir 82. More importantly, it delays the decay of vacuum in reservoir 82 upon engine shutdown, when manifold vacuum per se in port 80 quickly decays, to move the bleed valve 62 to an open position, as will be described in more detail later.

Servo vacuum chamber 68, on the other hand, is directly responsive to manifold vacuum in port 80 through a line 86 past a deceleration control valve assembly 88. Line 86 also contains an air port 90 connected by a branch line 58A to the air cleaner line 58, as shown. Opening or closing of port 90 is controlled by a movable valve 92 secured to the annular flexible diaphragm 94 ofa second vacuum controlled servo 96. A spring 98 normally biases the diaphragm and valve 92 to close port 90, the diaphragm dividing the shell 100 of the servo into an air chamber 102 and a manifold vacuum chamber 104. The air chamber is vented to the atmosphere through slots not shown. The vacuum chamber 104 is connected by a line 106 and line 78 to the manifold vacuum port 80.

The operation of the system is believed to be-clear from the above description and a consideration of the drawing and will not, therefore, be repeated in detail.

In brief, when the engine is shut down, atmospheric pressure exists in all portions of the carburetor including the idle channel 40, both servo vacuum chambers 68 and 70 of servo 64, and chambers 100 and 104 of servo 96. Accordingly, springs 76 and 98 force the valves 62 and 92 to close the air lines 52 and 54 and air port 90, respectively. Therefore, the carburetor is conditioned for a conventional operation in a known manner.

Upon engine startup, manifold vacuum acting in induction passage 12 is reflected through port 80 and lines 78, 106 and 86 in servo chambers 104, 68 and Because of orifice 84, the slow buildup in vacuum in reservoir 82 will cause a momentary delay in the vacuum buildup in chamber 70. However, this does not interfere with the operation at this time. The higher pres-' sure in chamber 70 merely aids the force of spring 76 to maintain valve 62 seated.

The preload force of spring 98 of servo 88 is so chosen that it will not permit movement of diaphragm 94 rightwardly by vacuum until the manifold vacuum is a very high value indicative of engine decelerating conditions of operating. Therefore, until this time, valve 92 will remain closed. again st port 90 by spring 98. Accordingly, manifold vacuum will flow through line 86 around valve 92 into chamber 78 to aid the spring 76 in maintaining valve 62 closed. Subsequently, when the vacuum in reservoir 82 reaches the level in chamber 68, the vacuum in chambers 68 and 70 will be balanced and permit the force of spring 76 alone to maintain valve 62 seated.

The flow of idle channel fuel and air through the carburetor at this time will occur in a'known manner, the vacuum signal acting at discharge port 44 and that area of the transfer slot 42 beneath the edge of the throttle valve 36 creating a pressure differential on the idle channel fuel orifice or jet, so that both air and fuel will be drawn through the idle channel 40 into the engine to maintain the desired engine idle or off idle speed.

The bleed valves 62 and 92 will remain in the positions shown for all conditions of operation of the engine except during deceleration and engine shutdown.

When the engine is decelerating, a very high manifold vacuum is developed in the lower portion of induction passage 14. Ordinarily, without the apparatus of the invention, this high signal acting on the idle channel discharge port 44 and the lower portion of transfer slot 42 would draw fuel and air through the idle channel into the engine cylinders. However, at this time, the scavenging of the exhaust gases or residual gases from the engine cylinders is very inefficient and burning incomplete. Accordingly, unburned hydrocarbons and other undesirable elements may pass out into the atmosphere. However, with the apparatus described, this does not occur.

The very high manifold vacuum signal during decelerating is sensed in port into servo chambers 104 and 68. The high vacuum only slowly acts in chamber 70 due to the restriction 84 and the lower vacuum level in reservoir 82. The high vacuum in chamber 104 is sufficient to overcome the force of spring 98 and open the air bleed valve 92. This immediately decays the high vacuum force in line 86 so that chamber 68 of servo 64 immediately decays to the essentially atmospheric pressure level prevelant at the air cleaner. This force, together with the vacuum in chamber- 70, is sufficient to overcome the force of spring 76 and immediately open valve 62 to connect the air cleaner air line 58 to ports 48 and 50 in idle channel 40. Immediately, the essentially atmospheric air pressure will now be acting on both sides of the fuel jet in the upper portion of idle channel 40 thereby destroying any vacuum signal on the fuel jet and terminating idle channel fuel flow.

At the same time, the very high vacuum signal in the manifold portion 12 of induction passage 14 draws air from the air cleaner through idle channel 40 out through the transfer slot 42 and discharge port 44 to not only lean whatever mixture is in the engine cylinders, but also to better scavenge the cylinders of whatever residual gases or products exist. This, therefore, reduces the output of undesirable elements to the atmosphere.

A similar situation occurs during engine shutdown, to prevent dieseling or afterrunning of the engine. When the engine is shut down, the intake manifold vacuum quickly decays towards an atmospheric pressure level. Even though decel valve 92 may be closing port 90, the decaying manifold vacuum is quickly reflected through line 86 to servo chamber 68. The orifice 84 prevents the quickly decaying manifold vacuum from immediately affecting the level of vacuum in reservoir 82. Accordingly, vacuum chamber 70 is still at a low vacuum level and permits the atmospheric pressure in chamber 68 to quickly overcome the force of spring 76 and open air bleed valve 62. This immediately connects idle channel 40 with the air cleaner air to immediately stop idle channel fuel flow to stop the engine from running, and immediately flows airthrough the idle channel below the throttle valve into the engine proper.

From the above, it will be seen that the invention provides a carburetor construction that automatically prevents passage of undesirable elements into the atmosphere during decelerating operations of the engine and automatically prevents afterrunning of the engine; in both instances by destroying the vacuum signal acting on the fuel jetin the engine idle air/fuel mixture channel and flowing air through the idle channel to the engine proper.

While the invention has been shown and described in its preferred embodiment, it will be clear to those skilled in the arts to which it pertains that many changes and modifications may be made thereto without departing from the scope of the invention.

l claim:

1. An engine anti-dieseling control comprising, in combination, an engine carburetor having aninduction passage open to atmospheric pressure at one end and adapted to be connected to an engine intake manifold at the opposite end so as to be subject to engine vacuum varying in level from ambient atmospheric pressure at engine shutdown to a maximum subatmospheric pressure level during engine deceleration operating conditions, a throttle valve rotatably mounted across the passage for controllingflow through the passage, an idle fuel/air mixture channel connected to the induction passage below the throttle valve so that the idle channel is subjected to manifold vacuum at all times to provide normal idle and beyond normal idle speed mixture flow, and control means responsive to engine shutdown conditions for admitting air into a portion of the idle mixture channel to decay the vacuum signal upstream thereof to a level essentially terminating idle channel fuel flow while maintaining an air flow to the engine, tothereby prevent engine dieseling, the idle channel having an air line connected to the idle channel at one end and to a source of air at essentially atmospheric pressure at the other end,- valve means in the air line movable between positions opening and closing the air line, and vacuum controlled servo means connected to the valve means for temporarily moving the valve means to an open position in response to decay of manifold vacuum upon engine shutdown to decay the idle channel vacuum and subsequently moving the valve means to a closed position to condition the carburetor idle channel for an engine start condition, the servo means including a movable diaphragm connected to the valve means and dividing the servo means into two channels, spring means biasing the diaphragm towards a valve means closed position, conduit means connecting the induction passage below the throttle valve to both chambers in parallel paths, a vacuum reservoir in one of the paths, and orifice means in the one path between the reservoir and induction passage to delay communication of change of pressure level in the passage to the reservoir whereby reservoir vacuum will open the air valve means upon engine shutdown decaying manifold vacuum in the other path, the subsequent decay of the reservoir vacuum permitting the spring means to bias the valve means closed.

2. A control as in claim l,'and second control means responsive to high manifold vacuum forces above a predetermined level that is indicative of engine decelerating conditions for admitting air into the idle mixtuated servo moved by vacuum for controlling the ap-,

plication of manifold vacuum to the first mentioned servo means. 7

3. A control as in claim 2, including a second line connecting manifold vacuum to the first and second servos to act thereon, the second line containing an air port controlled ,by a second valve movably connected to the second servo means and operable upon the attainment of a high vacuum force indicative of.engine decelerating conditions to move the second valve open to decay vacuum in the second line to the first servo whereby the first servo moves the first valve to open to flow air into the idle channel.

4. An engine anti-dieseling and deceleration control comprising, in combination, an engine carburetor having an air/fuel induction passage, an air cleaner assembly discharging clean air atessentially atmospheric pressure into one end of the passage, the other end of the passage being connected to the intake manifold of the engine so as to subject the passage to changes in manifold vacuum, a throttle valve rotatably mounted for movement across the passage, an idle air/fuel mixture channel connected to the induction passage at a point below the throttle valve so as to be subject to the manifold vacuum at all times to provide an idle speed and off idle speed air/fuel mixture flow, and control means to regulate the flow of fuel through the idle channel during engine decelerating conditions and at engine shutdown to minimize the emission of undesirable elements into the atmosphere, I

the control means including a first air port operably connected to the idle mixture channel, first conduit means connecting the port of the air cleaner for the decay at times of a portion of the channel vacuum by passage of air from the air cleaner to the port, a first manifold vacuum sensitive valve movably associated with the port for opening and closing the same, spring means biasing the valve in a closed direction,

second conduit means operably connecting manifold vacuum to the first valve for-moving the same in the opposite direction, a second air port in the second conduit means connected to the air cleaner for at times decaying the vacuum in the second conduit means, a second manifold vacuum sensitive valve associated with the second port for controlling opening and closing of the same, second spring means biasing the second valve closed,

and a vacuum reservoir operably connected to the first valve for at times moving the first valve against the spring bias, an increase in manifold vacuum above a predetermined force indicative of engine decelerating conditions moving the first and sec ond valves by vacuum to open both air ports and decay the idle channel vacuum and flow air to the engine, the decay of vacuum upon engine shutdown effecting decay of the idle channel vacuum by movement of the first valve by the reservoir vacuum to open the first air port, to thereby prevent engine dieseling.

5. A control as in claim 4, including a first vacuumcontrolled servo connected to the first valve and having a first vacuum chamber connected to the second conduit means and a second vacuum chamber connected to the vacuum reservoir.

6. A control as in claim 4, including a vacuum sensitive servo connected to the second valve and movable by manifold vacuum acting thereagainst above the predetermined level to oppose the second spring means and open the second valve, to admit air to the second conduit means.

7. A control as in claim 4, including first and second vacuum controlled movable diaphragm type servos connected respectively to the first and'second valves, the first servo having a first vacuum chamber connected to the second conduit means and a second vacuum chamber connected to" the vacuum reservoir, movement of the second valve by vacuum above the predetermined level opening the second air port and decaying the vacuum in the second conduit means, permitting reservoir vacuum to open the first air port to decay the idle channel vacuum to reduce emissions.

8. A control as in claim 7, including third conduit means connecting the manifold vacuum to the reservoir, and an orifice in the third conduit means between the reservoir and manifold to delay change in pressure level of the reservoir upon change in pressure level of the manifold.

9. A carburetor having an induction passage exposed to engine manifold vacuum and an idle system fuel/air mixture channel connected thereto, a supplemental air bleed port in the idle system channel, a valve movable by vacuum to an open position to admit and by spring means to a closed position to block flow of air through the port into the channel, and means operable upon en'- gine shutdown for initially effecting movement of the valve to open the port to decay the idle system vacuum to terminate idle system fuel flow, and subsequently automatically after a timed delay effecting movement of the valve to close the port, the last mentioned means including a vacuum servo connected to the valve and having first and second conduit means conducting manifold vacuum from the induction passage to opposite sides respectively of the servo to obtain a force balanced condition, spring means aiding the fluid forces on one side and biasing the valve closed, the first conduit means including a vacuum accumulator reservoir and a flow restriction between the reservoir and the induction passage connected to manifold vacuum whereby engine shutdown decaying manifold vacuum to a pressure above the level of reservoir vacuum effects movement of the valve against the spring means to admit air into the port until subsequent decay of the reservoir vacuum again obtains a net force condition permitting spring movement of the valve to block the port and condition the idle system channel for normal operation. 

1. An engine anti-dieseling control comprising, in combination, an engine carburetor having an induction passage open to atmospheric pressure at one end and adapted to be connected to an engine intake manifold at the opposite end so as to be subject to engine vacuum varying in level from ambient atmospheric pressure at engine shutdown to a maximum subatmospheric pressure level during engine deceleration operating conditions, a throttle valve rotatably mounted across the passage for controlling flow through the passage, an idle fuel/air mixture channel connected to the induction passage below the throttle valve so that the idle channel is subjected to manifold vacuum at all times to provide normal idle and beyond normal idle speed mixture flow, and control means responsive to engine shutdown conditions for admitting air into a portion of the idle mixture channel to decay the vacuum signal upstream thereof to a level essentially terminating idle channel fuel flow while maintaining an air flow to the engine, to thereby prevent engine dieseling, the idle channel having an air line connected to the idle channel at one end and to a source of air at essentially atmospheric pressure at the other end, valve means in the air line movable between positions opening and closing the air line, and vacuum controlled servo means connected to the valve means for temporarily moving the valve means to an open position in response to decay of manifold vacuum upon engine shutdown to decay the idle channel vacuum and subsequently moving the valve means to a closed position to condition the carburetor idle channel for an engine start condition, the servo means including a movable diaphragm connected to the valve means and dividing the servo means into two channels, spring means biasing the diaphragm towards a valve means closed position, conduit means connecting the induction passage below the throttle valve to both chambers in parallel paths, a vacuum reservoir in one of the paths, and orifice means in the one path between the reservoir and induction passage to delay communication of change of pressure level in the passage to the reservoir whereby reservoir vacuum will open the air valve means upon engine shutdown decaying manifold vacuum in the other path, the subsequent decay of the reservoir vacuum permitting the spring means to bias the valve means closed.
 2. A control as in claim 1, and second control means responsive to high manifold vacuum forces above a predetermined level that is indicative of engine decelerating conditions for admitting air into the idle mixture channel to decay the vacuum therein, the second control means including a second manifold vacuum actuated servo moved by vacuum for controlling the application of manifold vacuum to the first mentioned servo means.
 3. A control as in claim 2, including a second line connecting manifold vacuum to the first and second servos to act thereon, the second line containing an air port controlled by a second valve movably connected to the second servo means and operable upon the attainment of a high vacuum force indicative of engine decelerating conditions to move the second valve open to decay vacuum in the second line to the first servo whereby the first servo moves the first valve to open to flow air into the idle channel.
 4. An engine anti-dieseling and deceleration control comprising, in combination, an engine carburetor having an air/fuel induction passage, an air cleaner assembly discharging clean air at essentially atmospheric pressure into one end of the passage, the other end of the passage being connected to the intake manifold of the engine so as to subject the passage to changes in manifold vacuum, a throttle valve rotatably mounted for movement across tHe passage, an idle air/fuel mixture channel connected to the induction passage at a point below the throttle valve so as to be subject to the manifold vacuum at all times to provide an idle speed and off idle speed air/fuel mixture flow, and control means to regulate the flow of fuel through the idle channel during engine decelerating conditions and at engine shutdown to minimize the emission of undesirable elements into the atmosphere, the control means including a first air port operably connected to the idle mixture channel, first conduit means connecting the port of the air cleaner for the decay at times of a portion of the channel vacuum by passage of air from the air cleaner to the port, a first manifold vacuum sensitive valve movably associated with the port for opening and closing the same, spring means biasing the valve in a closed direction, second conduit means operably connecting manifold vacuum to the first valve for moving the same in the opposite direction, a second air port in the second conduit means connected to the air cleaner for at times decaying the vacuum in the second conduit means, a second manifold vacuum sensitive valve associated with the second port for controlling opening and closing of the same, second spring means biasing the second valve closed, and a vacuum reservoir operably connected to the first valve for at times moving the first valve against the spring bias, an increase in manifold vacuum above a predetermined force indicative of engine decelerating conditions moving the first and second valves by vacuum to open both air ports and decay the idle channel vacuum and flow air to the engine, the decay of vacuum upon engine shutdown effecting decay of the idle channel vacuum by movement of the first valve by the reservoir vacuum to open the first air port, to thereby prevent engine dieseling.
 5. A control as in claim 4, including a first vacuum-controlled servo connected to the first valve and having a first vacuum chamber connected to the second conduit means and a second vacuum chamber connected to the vacuum reservoir.
 6. A control as in claim 4, including a vacuum sensitive servo connected to the second valve and movable by manifold vacuum acting thereagainst above the predetermined level to oppose the second spring means and open the second valve, to admit air to the second conduit means.
 7. A control as in claim 4, including first and second vacuum controlled movable diaphragm type servos connected respectively to the first and second valves, the first servo having a first vacuum chamber connected to the second conduit means and a second vacuum chamber connected to the vacuum reservoir, movement of the second valve by vacuum above the predetermined level opening the second air port and decaying the vacuum in the second conduit means, permitting reservoir vacuum to open the first air port to decay the idle channel vacuum to reduce emissions.
 8. A control as in claim 7, including third conduit means connecting the manifold vacuum to the reservoir, and an orifice in the third conduit means between the reservoir and manifold to delay change in pressure level of the reservoir upon change in pressure level of the manifold.
 9. A carburetor having an induction passage exposed to engine manifold vacuum and an idle system fuel/air mixture channel connected thereto, a supplemental air bleed port in the idle system channel, a valve movable by vacuum to an open position to admit and by spring means to a closed position to block flow of air through the port into the channel, and means operable upon engine shutdown for initially effecting movement of the valve to open the port to decay the idle system vacuum to terminate idle system fuel flow and subsequently automatically after a timed delay effecting movement of the valve to close the port, the last mentioned means including a vacuum servo connected to the valve and having first and second conduit means conducting manifold vacuum from the induction pasSage to opposite sides respectively of the servo to obtain a force balanced condition, spring means aiding the fluid forces on one side and biasing the valve closed, the first conduit means including a vacuum accumulator reservoir and a flow restriction between the reservoir and the induction passage connected to manifold vacuum whereby engine shutdown decaying manifold vacuum to a pressure above the level of reservoir vacuum effects movement of the valve against the spring means to admit air into the port until subsequent decay of the reservoir vacuum again obtains a net force condition permitting spring movement of the valve to block the port and condition the idle system channel for normal operation. 